energy in motion
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heat transfer
• energy that moves from higher temperature objects to lower temperature objects • can happen in three ways: conduction, convection, radiation
conduction
• involves heat being transferred through direct contact • hotter, faster moving particles will collide with the colder, slower moving particles, transferring some kinetic energy in the process • the average kinetic energy of the hotter paricles will reduce, while the avaerage kinetic energy of the colder particles will increase, until they reach the same value • occurs best with solids
what happens with solids in conduction
• particles are tightly packed and constantly vibrating • easy for neighbouring particles to collide and transfer kinetic energy
what happens with liquids in conduction
• particles are arranged randomly and sliding into eachother
what happens with gases in conduction
• particles are very far apart • collide occasionally • difficult for kinetic energy to be transferred
substances good at conducting heat (thermal conductors) poor at conducting heat (thermal insulators) what are good thermal conductors?
• metals due to their delocalised electrons (detached) enabling more collisions
convection
• involves the bulk movement of hot particles from one place to another within a fluid (liquid,gas)
forced convection
• hot fluid is purposefully moved via a force of some kind e.g a fan-forced heater blows hot air onto your feet
natural convection
• hot fluid expands, which increases its volume, which decreases its density, which causes it to rise e.g water that is being boiled in a big pot
What is conduction in simple?
Heat transfer by direct contact of particles.
How is heat transferred in conduction?
Hot, fast particles collide with cold, slow ones → kinetic energy transfer.
What happens to the temperatures of hot and cold particles? conduction
Hot particles lose energy, cold particles gain energy → eventually equalise.
In which states does conduction occur best?
Best in solids, less in liquids, least in gases.
Why is conduction most effective in solids?
Particles are tightly packed, collisions/energy transfer easier.
Why is conduction less effective in liquids?
Particles are random/sliding → fewer effective collisions.
Why is conduction least effective in gases?
Particles far apart → rare collisions.
What are thermal conductors and insulators?
Conductors = transfer heat well. Insulators = poor at transferring heat.
Why are metals good thermal conductors?
Delocalised electrons carry energy quickly through collisions.
What is convection?
Transfer of heat by the bulk movement of hot fluid particles (liquid or gas).
In what states does convection occur?
Only in fluids (liquids + gases).
What are the two types of convection?
Forced convection and Natural convection.
What is forced convection? (+ example)
Hot fluid is moved by an external force (e.g. fan-forced heater blowing hot air).
What is natural convection? (+ example)
Hot fluid expands → less dense → rises. Cooler fluid sinks, forming convection currents (e.g. boiling water).
What is thermal radiation?
Heat transfer by electromagnetic waves emitted by all objects.